1. Field of the Invention
This invention relates generally to a plasma generating apparatus for use in plasma spectroscopy, and more specifically to an improved apparatus having an inductively coupled plasma torch and electrothermal atomizer.
2. Background Information
In inductively coupled plasma (ICP) spectroscopy, a procedure is utilized in which a sample of analyte (a material to be analyzed) is vaporized or atomized and passed into a plasma chamber where its spectra may be observed and analyzed.
As shown in FIG. 1, prior art ICP spectroscopy systems have typically included an electrothermal atomizer 20 interconnected with a separate ICP torch 30 by means of a long pipe, conduit or similar device 35. Typically, the atomizer 20 is composed of a large bell-shaped glass ]ar 22 which encases a carbon rod heater element 24 extending between two electrodes 26. The glass jar 22 may be lifted, whereupon a sample of analyte may be placed in the heater element 24 and the jar 22 replaced. The analyte is then heated to induce atomization of the analyte. As atomization commences, a supply of carrier gas 28 (such as argon) is flowed over the electrodes where it mixes with the atomizing analyte. The mixture flow within the bell jar 22 passes through the pipe or conduit 35 to the ICP torch 30.
A conventional ICP torch 30 is typically formed of quartz glass and includes intricate ducting. A main duct 31 is provided into which the analyte mixture flow may pass. An auxiliary duct 32 is also provided through which an auxiliary flow of argon or a similar gas may be introduced within a quartz glass containment wall 34. In addition, a coolant duct 33 is provided into which a coolant gas may be introduced. A conventional ICP torch also has a radio frequency (RF) induction coil 36 which is wrapped about the containment wall 34. When energized, the induction coil 36 generates plasma out of the gas introduced through the auxiliary duct 32. Upon injection of the analyte mixture from the atomizer 20 into the of main duct, the mixture passes through the injector tip 37 of the torch 30 where it mixes with the plasma. The analyte mixture is thus heated up to the temperature of the plasma, wherein most of the molecules are ionized so that the mixture may be analyzed.
Another common prior art spectroscopy system provides for the interconnection of a spray chamber rather than an atomizer to an ICP torch by means of a pipe or conduit similar to the one described above. A liquid analyte then may be placed within the spray chamber where it may be converted into a fine spray for introduction to the ICP torch 30.
Prior art conventional ICP spectroscopy systems such as those described above typically suffer from several disadvantages. First and foremost, a myriad of locations exist within the system at which analyte may accumulate as condensation after completion of an analysis. This causes the system to have a "memory" that may cause contamination of subsequent analytes. These locations include, but are not limited to, (1) the walls of the atomizer 20; (2) the pipe or conduit 35 interconnecting the atomizer 20 or a spray chamber and the ICP torch 30; (3) the injector tip 37 of the ICP torch 30; and (4) the quartz glass containment wall 34 of the ICP torch 30.
A further disadvantage of the above-described systems is the severe signal loss which occurs between the atomizer 20 or spray chamber and the ICP torch 30 as a result of condensation. Since the path between the atomizer 20 or spray chamber and the containment wall 34 of the ICP torch 30 is long and condensate accumulates along the entirety of this path, only a relatively small portion of the analyte ever reaches the plasma chamber to be analyzed, creating a greater than desirable signal loss in results.
A further disadvantage of the above-described prior art ICP spectroscopy systems is their inadaptability to robotic sample loading. Vaporization of the analyte typically occurs under sealed glassware. Robotic systems, however, are not typically suited to operate in environments where glassware is encountered.